This invention relates to electron microscopes, and more particularly to a method and apparatus for producing improved images from such microscopes, both with regard to electron-energy filtered imaging of an object with a transmission electron microscope, and with regard to the electron-energy filtered imaging of an object diffraction pattern with a transmission electron microscope.
The invention deals with microscopes in which the electron beams proceeding from a source of electrons illuminate the object, and the object is then imaged by a first imaging stage into the input-image plane of an imaging electron-energy spectrometer, and at the same time the source of electrons is imaged into the input crossover point of the spectrometer. Furthermore, the electrons of a selectable energy range are filtered out in the spectrometer, and the filtered image of the object, from the output-image plane of the spectrometer, is then imaged into the final image-plane. The invention furthermore concerns a corresponding method and apparatus for object diffraction patterns.
One important advantage of a transmission electron microscope having an imaging electron-energy spectrometer is that, by the selection of electrons of a given energy range, a new form of high contrast object imaging is obtained, that is not possible in the case of conventional instruments. This can be noted very clearly in images of very thin unstained specimens having a selected energy loss (.DELTA.E) within the range of about 100 to 200 eV. However, an object image produced exclusively by elastically scattered electrons (energy loss .DELTA.E=0) so that all inelastically dispersed electrons (.DELTA.E&gt;0) are eliminated, is also clearly improved in contrast, as compared with the unfiltered image.
Another important advantage of a transmission electron microscope having an imaging electron-energy spectrometer resides in the possibility of obtaining element-specific enhancement of object images (i.e., element distribution images) simultaneously over a relatively large selected specimen area in such manner that the energy range permitted to pass through by the electron-energy spectrometer corresponds to an element-specific interaction of the transmitted electrons with the object, and corresponds, for instance, to a K- or L- or M- absorption in the atomic shell of the element. In this way, not only qualitative but also (upon measurement of the intensity ratios and elimination of the background) quantitative atomic distribution iamges in thin objects (thickness equal to or less than 30 nm) can be obtained with very high spatial resolution (approximately equal to 0.5 nm) and with very high sensitivity of elemental detection (approximately equal to 2.times.10.sup.-21 g), such as were not obtained heretofore by any other method of analysis. Maximum obtainable spatial resolution and sensitivity of detection of elements are of great importance both for biological and medical research and for the science of materials.
In addition, in the case of electron diffraction patterns, an imaging electron-energy spectrometer produces sharper images of the diffraction pattern.
Federal Republic of Germany Pat. No. 2,028,357 and its counterpart U.S. Pat. No. 3,624,393 of E. Torquebiau, granted Nov. 30, 1971, disclose an electron microscope which makes possible a filtering not only of the object image but also of the diffraction image. To accomplish this, an enlarged image of the diffraction pattern is produced by a first imaging stage consisting of three lenses in the input-image plane of the spectrometer (designated B.sub.2 in the patent) and an object image is produced in the input crossover plane (designated B.sub.1 in the patent). The disadvantages of this known device are that only one magnification is possible for the filtered diffraction pattern and that only a slight variation in magnification is possible for filtered object images, which variation is not indicated in the patent, and in which the excitation of the objective lens must be reset for the focusing of the image.
From the literature, a few other electron microscopes having imaging electron-energy spectrometers are known. In the Journal of Ultrastructure Research, volume 72 (1980), pages 336-348, there is an article by F. P. Ottensmeyer and J. W. Andrew, which describes an electron microscope having an imaging spectrometer between an objective lens with first and second intermediate lenses and a projective lens. This imaging spectrometer consists of an input lens, the actual prism-mirror-prism spectrometer (Castaing type), and an output lens.
This arrangement has the most flexible imaging optics of any arrangement known prior to the present invention. The change in magnification takes place essentially by changing the excitation on the two intermediate lenses. In this connection, however, the position of the image of the object and the image of the electron source (crossover) in front of the spectrometer are changed. By corresponding excitation of the input lens of the spectrometer, the crossover must be brought to the correct place for the actual spectrometer, and the position of the image of the object is corrected by refocusing the objective lens. The same procedure is necessary when the back focal plane of the objective is imaged, in order to image an electron diffraction pattern by varying the excitation of first and second intermediate lenses, into the spectrometer. The limiting of the imaging aperture of the spectrometer is effected by a field limiting aperture in the first intermediate image after the objective lens.
This known combination of electron microscope and electronenergy spectrometer, described in the above Journal article, has the following disadvantages:
1. The magnification range is limited to medium and high magnification (&gt;3000 X); it is pushed upward by a factor of two as compared with the original electron microscope. The possible variation in magnification amounts to about 150:1.
2. For each magnification step, the input lens of the spectrometer must first be set in iteration steps in order to maintain the position of the crossover, whereupon the objective lenses are refocused for the focusing of the image, and the spectrometer adjusted laterally and rotationally, etc. Accordingly, a change in magnification is very time-consuming and thus leads to irreparable radiation damage in the case of objects which are sensitive to radiation. As a consequence, a single preadjusted magnification is used per operating session, e.g., morning or afternoon, etc.
3. The effective aperture for the spectrometer, i.e., the image of the field limiting aperture in the input-image plane, changes upon change in the magnification. Accordingly, for each magnification setting, a corresponding field limiting aperture must be selected in order to obtain optimum adaptation of spectrometer energy resolution to the desired image format (photograph format). This is practically impossible in view of the large number of magnification steps required.
4. The imaging electron-energy spectrometer of the prism-mirror-prism type (Castaing type) has clear aberrations which limit the energy-resolution capability. It is known, to be sure, from an article by J. W. Andrew, F. P. Ottensmeyer, and E. Martell in the papers of the Ninth International Congress on Electron Microscopy, Toronto, 1978, volume 1, pages 40-41, that by using curved entrance and exit surfaces of the prism, a reduction in aberrations is possible as compared with using flat or plane entrance and exit surfaces. However, no information is given in this article as to advantageous specific embodiments for such curved surfaces.
The object of the present invention is, therefore, to obtain, in a transmission electron microscope with an imaging electron-energy spectrometer, the largest possible range of magnification without the necessity of readjustment or refocusing upon a change in the magnification.
Another object is to provide, in such a microscope, a spectrometer-aperture diaphragm which is independent of the magnification, for selection of the energy resolution with optimum adaptation to the image-limiting area of the specific detector.
Still another object of the invention is to make possible, with the same transmission electron microscope, the imaging of energy-filtered diffraction patterns with a variation in magnification of at least 3:1.
A further object is to improve the energy resolution of the electron-energy spectrometer with prism-mirror-prism system.